DAMASCUS, Syria — Secretary of State James A. Baker III announced here Thursday that Syria has agreed to an American plan for a Middle East peace conference, but he raised new uncertainty about the details of the still-secret American proposal.

"It is apparent to me that Syria has made a very important decision," Baker told a news conference here after a 2 1/2-hour meeting with Syrian President Hafez Assad.

"Syria has agreed to the proposals we have made. . . . It gives us something to work with, and we are going to try to work with it on this trip to try to build the cause of peace."

Last Monday, after receiving a private letter from Assad, President Bush termed Syria's response a "positive breakthrough." But until the secretary's visit here Thursday, it was unclear whether Syria had actually agreed to the U.S. proposal.

At his press conference, Baker appeared to indicate that the United States may have made some concessions to Syria that would give the United Nations a greater role at the proposed peace conference than the Bush Administration had earlier envisioned.

One of the main sticking points that has held up agreement on a peace conference is the role of the United Nations. Syria wanted the United Nations to sponsor the peace conference, or at least to be an active participant in it. Israel objected to any significant U.N. participation on grounds that the organization has for years shown it is biased against the Jewish state.

In an effort to work out a compromise, the Bush Administration had suggested that the United Nations take part in the peace conference as a "silent observer."

But Baker on Thursday repeatedly avoided using the word "silent." And he left the impression that the U.N. representative at the proposed conference will be able to participate, at least to some extent.

"The (U.N.) representative would be an observer. . . . He will be able to communicate with the participants and the sponsors," the secretary of state said. The United States and the Soviet Union would sponsor the conference.

One senior Administration official later explained that under the U.S. plan that Syria accepted, the U.N. representative might ask questions or talk to other participants but that "he (the U.N. representative) is not there to give a speech."

Baker, who is due to visit Israel on Sunday, declined to speculate on how the government of Prime Minister Yitzhak Shamir might respond to the U.S. proposal in its current form. "I cannot prejudge what their reactions will be," he said.